1. Field of the Invention
The present invention relates generally to NTSC high resolution television converting apparatus and, more particularly, to NTSC high resolution television converting apparatus for enabling a video signal based on an NTSC system (hereinafter abbreviated as NTSC signal) to be imaged onto a high resolution television monitor having an aspect ratio of 16:9.
2. Description of the Background Art
In recent years, as color television receivers require larger-scale screens, degradations in picture quality due to the NTSC system becomes a significant problem. In order to solve this problem, a high resolution television system is proposed and is in practical use as a new video service including information approximately five times as much as the information included in the present NTSC system and providing a highly improved picture quality. The high resolution television system has originally been considered as a new television system unrelated with existing systems and hence is incompatible with the present NTSC system as it is. It is inefficient, however, to own two separate receivers: the one for NTSC and the other for a high resolution television at the stage of propagation of the high resolution television system. It is thus desirable that both the present NTSC system and the high resolution television system can be received by employing only either one of those receivers.
FIGS. 1A and 1B are diagrams illustrating an example of a display in the case where a video having an aspect ratio of 4:3 (a video such as of an NTSC signal) is imaged on a high resolution television monitor having an aspect ratio of 16:9. If video information with the aspect ratio of 4:3 are all displayed on the high resolution television monitor with the aspect ratio of 16:9, blank spaces 101 are provided at both the left and right sides of the monitor by time-base compression of a video period in a horizontal direction. A side level signal (a gray level in general) is inserted into the spaces 101 to prevent the burning of a fluorescent material applied on a CRT. Further, as shown in FIG. 1B, if the video is enlarged so that it can be imaged on the entire display screen of the high resolution television monitor with the aspect ratio of 16:9, the video with the aspect ratio of 4:3 has its top and bottom portions 102 cut away. Thus, the display method shown in FIG. 1A is indispensable when the video is imaged on the high resolution television monitor.
FIG. 2 is a block diagram showing one example of a converter for displaying a video in the display manner shown in FIG. 1A. With reference to FIG. 2, an NTSC signal is applied to an input terminal 111. The NTSC signal has, as already known, 525 scanning lines per frame, an interlace of 2:1, a field frequency of 59.94 Hz and a line frequency of 15.73 kHz. This NTSC signal is applied to a signal processor 112 for processing of an EDTV (Extended Definition TV) signal, to be converted into a signal (EDTV signal) having 525 scanning lines per frame, a sequential scanning of 1:1, a frame frequency of 59.94 Hz and a line frequency of 31.468 kHz.
EDTV signal processor 112 serves as a signal converter for achieving a higher picture quality while maintaining the compatibility of an NTSC signal. More specifically, EDTV signal processor 112 performs signal processings such as motion adaptive three-dimensional Y/C separation and sequential scanning conversion.
FIG. 3 is a diagram illustrating the waveform of an output signal applied from EDTV signal processor 112 shown in FIG. 2 only during one horizontal scanning period. A video signal subjected to signal conversion for EDTV has its time base in a horizontal scanning direction compressed by a time-base compression processor 113. This time-base compression causes a blank period in one horizontal scanning period. Thus, a signal switch 114 is provided at the succeeding stage of time-base compression processor 113, and a side level signal generating source 115 is also provided. The time-base compressed video signal and side level signal are appropriately switched by signal switch 114, so that the EDTV signal shown in FIG. 3(b) is output.
As one of methods for time-base compression, a method employing a FIFO type line memory is provided. When a compression ratio is n (n&lt;1), a video signal corresponding to one horizontal scanning period, which is a write clock signal in the line memory, is first written into the line memory. The video signal written in the line memory is then read out in response to a read clock signal having a frequency 1/n times that of the write clock signal.
The problem to be discussed here is to which value the compression ratio n should be set. As far as the aspect ratio of the signals is simply concerned, since a video of 12:9 in width to length (the aspect ratio of 4:3) is imaged on the screen of 16:9 in width to length (the aspect ratio of 16:9), the compression ratio n should be n=12/16=3/4.
This value of compression ratio n is, however, given irrespectively of a blanking period of a video signal and an overscanning rate of the monitor. As a practical matter, if an NTSC signal is imaged on the high resolution television monitor with the aspect ratio of 16:9, a perfect circle must be reproduced accurately as a perfect circle. If the above-described compression ratio 3/4 is adopted, the original perfect circle becomes slightly oval. Moreover, since a basic clock signal of EDTV signal processor 112 is 8.multidot.f.sub.sc (=28.6 MHz) in general, the sampling number of one horizontal scanning period is 910. In time-base compression processor 113, the video signal is time-base compressed only during a video period, and thus the length of one horizontal scanning period is invariable, as apparently seen from FIG. 3(a) and (b). In the line memory provided in time-base compression processor 113, an NTSC signal is written in response to a clock signal of 8.multidot.f.sub.sc and read out at the frequency of a read clock signal of 8.multidot.f.sub.sc .times.1/n. Accordingly, if one horizontal scanning period is sampled at this frequency, the sampling number is 910 /n. The above-described value of 910/n should be an integer when the compression ratio n is selected. If the value 910/n is not an integer, the video is out of phase one by one line, so that vertical lines are in the form of steps.
As described above, when the compression ratio is 3/4, the value 910/n is 910/n=910.times.4/3.apprxeq.1213.33. Since the obtained value is not an integer, the foregoing out-of-phase is generated.